I am thinking that the reason that the glass feet are so rare is that they did get broken, a lot. They may have been successful but Singer may have decided that it was not an improvement to continue with after purchasing Wheeler & Wilson for cost reasons.
At some point the sewing machine owner likely discovered that the machine would sew without the glass feet inserts. Likely the glass feet were more readily available a hundred years ago. Like Charlee, I am hesitant to sew on mine as I have only one of the glass feet.